1. Field of the Invention
This invention relates to an improved method and system for cementing the casing of an oil well in a bore hole.
2. Background
Currently, conventional oil wells require that drilling equipment be used to create a bore hole to a desired depth. The drilling equipment is removed and replaced with a casing string that is secured in the bore hole by cement. Generally, the casing is run down the bore hole with a closed end, typically with a guide shoe and a float collar.
A simple guide shoe, such as an open-end collar with a molded nose, is attached at the end of the casing string. This shoe directs the casing away from ledges in the bore hole and minimizes caving of the sidewalls of the bore hole. At a point one to three joints above the guide shoe is the float collar that prevents fluids from entering the casing string. This closed end allows the casing string to be floated down the bore hole, at the same time increasing pressure on the bore hole walls by the displacement of mud and fluid from the center of the bore hole. This pressure can cause a strain on weaker formations to break down or plant the casing above the desired depth.
If the close-ended casing string has become planted or mud is needed to be cleared from around the casing, the casing string is reciprocated or rotated. This reciprocation may result in a swab-surge effect on the bore hole walls. If the casing string is hung up on an obstruction in the bore hole, the casing is either reciprocated and rotated through the obstruction or the casing is pulled out of the bore hole so that drilling equipment can re-enter to remove the obstruction. It is only after the closed-end casing has reached the desired depth in the bore hole may the cementing process be carried out.
A commonly-used cementing method for close-ended casing requires only a top and bottom cementing plug with a cement slurry interposed. The bottom plug is pressed into the casing to clear mud and debris from the inside of the casing string to prevent contamination of the cement slurry. The top plug forces down the cement slurry against the bottom plug. When the bottom plug reaches the float collar, the pressure from the cement slurry ruptures the upper diaphragm on the top of the bottom plug and passes through the hollow center and down through the lower diaphragm out, into and around the guide shoe and up and around the annular space between the bore hole and casing. The drawback to this case cementing method is the requirement that the casing be close-ended. This close-ended configuration can damage the formation and inhibit the casing string from reaching its desired depth. This cementing method requires additional equipment and may require a protracted rig time to set the well. It is to this end that the present invention has been developed to provide a system and method for cementing casing in a bore hole.